This invention is a method for the production of grain oriented electrical steel, and more particularly, grain oriented silicon electrical steel, starting from a thin slab. In one embodiment, this method refers to a product formation route which enables efficient production with better yield and wider process control tolerance.
The prior art describes variations of the product and the process to make many variations of electrical steels. Patents issued to Hadfield starting in 1903 (such as U.S. Pat. Nos. 745,829; 836,762; 836,754; 836,755; 836,756) are among the earliest patents in the field of this invention. Such patents describe the magnetic performance of electrical steels and the composition for making electrical steels with methods using the technology available around the year 1900. Patents assigned to Armco Steel Corporation, Ohio and the General Electric Company, New York, from the year 1950 onward (such as U.S. Pat. Nos. 2,535,420; 2,599,340; 2,867,558) describe variations and improvements to the product and process to incorporate continuous manufacturing operations and improved process control.
Traditionally, electrical steels have been made by casting ingots or slabs that are 200-250 mm thick. In such processes large oriented grain growth is obtained in the final stages of the process at a step referred to High Temperature Anneal (HTA) where the steel strip is held at elevated temperatures of around 1200° C. for an extended period of time. In the HTA step certain chemical systems inhibit the growth of general or normal grains while allowing the large oriented grains to grow. These chemical systems are referred to as the “inhibitor system” for a given process. In the past, electrical steels have used one of two inhibitor systems, which are the (a) sulfide-manganese system, and (b) nitride-aluminum system.
The sulfide-manganese system has been known to result in high quality electrical steels but it has several major drawbacks. It requires high temperature reheat of the slab to re-dissolve the inhibitor species which tend to escape from the iron crystal grains when the slab is solidifying after casting. It also requires tight process control since sulfur has a high propensity to escape from the iron crystal grains. Moreover, when sulfur rich chemical species collect at the grain boundaries they cause the problem of red-shortness or hot-shortness which results in cracking and breakage of steel strips and results in yield loss.
The nitride-aluminum system has been used to make electrical steels with a lower reheat temperature of the slab. But since a thick slab (200-250 mm) takes a considerable time to solidify it still provides an environment in which the inhibitor species escape from the iron crystal grains. As such the process has a few drawbacks: 1) it still requires an energy intensive reheat step; and 2) the inhibitor species have a propensity to chemically combine with other impurities present in the steel and thus result in lower levels of inhibitors at the HTA step and also create new impurities that compromise the performance and properties of the finished electrical steel.
In the recent past attempts have been made to produce electrical steels using sulfide-manganese inhibitor systems along with thin slab casting technology which casts a slab from 20-80 mm (see for example U.S. Pat. No. 6,296,719). This offers the benefit of low energy consumption since the slab can be rolled to final gauge from a much smaller starting thickness. It also offers the benefit of obtaining favorable microstructure in the slab. But since it is based on sulfide-manganese inhibitor systems, the process still requires slab reheat to about 1300° C. and still is susceptible to the drawbacks which are characteristic of such systems.